1. Field of the Invention
The present invention relates to a window glass antenna for use on a window glass panel such as an automobile window glass panel.
2. Description of the Prior Art
One conventional window glass antenna known from Japanese laid-open patent publication No. 57-188102 is shown in FIG. 13 of the accompanying drawings.
The window glass antenna shown in FIG. 13 is used to receive both AM (amplitude-modulated) and FM (frequency-modulated) broadcast radiowaves, and capable of diversity reception of FM radiowaves.
The window glass antenna, generally designated by the reference numeral 100 in FIG. 13, comprises a plurality of defrosting hot wires 102 disposed on a window glass panel 101 and extending horizontally thereon, an inverted T-shaped antenna wire 103 disposed on the window glass panel 101 in an unheated region above the defrosting hot wires 102, a capacitive coupling wire 104 disposed on the window glass panel 101 between the defrosting hot wires 102 and the inverted T-shaped antenna wire 103 and providing a capacitive coupling therebetween, the capacitive coupling wire 104 having a low impedance at relatively low frequencies such as AM broadcast frequencies, power supply lines 111 connected between the defrosting hot wires 102 and a hot-wire power supply 110, and high-frequency blocking coils 112 inserted in the power supply lines 111 for providing a high impedance with respect to signals having relatively low frequencies such as AM broadcast frequencies. The defrosting hot wires 102 have a horizontal length of L11. The-power supply lines 111 have a length L10 extending from the high-frequency blocking coils 112 to the defrosting hot wires 102.
The inverted T-shaped antenna wire 103 includes a vertical wire section 103a having an upper end 103b connected to an auxiliary antenna wire 105 which is in turn connected through a first feeding point 106 to a first high-frequency amplifier 113. AM/FM (main) broadcast radio wave signals are received from the inverted T-shaped antenna wire 103 through the auxiliary antenna wire 105, the first feeding point 106, and the first high-frequency amplifier 113. The defrosting hot wires 102 have right-hand ends (as viewed in FIG. 13) connected to a bus 107 which includes a second feeding point 108 connected to a second high-frequency amplifier 114. FM (auxiliary) broadcast radio wave signals are received from the defrosting hot wires 102 through the bus 107, the second feeding point 108, and the second high-frequency amplifier 114.
AM broadcast radio wave signals of the AM/FM (main) broadcast radio wave signals which have been amplified by the first high-frequency amplifier 113 are supplied to an AM antenna terminal of an AM/FM receiver (not shown). FM (main) broadcast radio wave signals of the amplified AM/FM (main) broadcast radio wave signals are supplied to one input terminal of a selector switch (not shown).
The FM (auxiliary) broadcast radio wave signals which have been amplified by the second high-frequency amplifier 114 are supplied to the other input terminal of the selector switch.
The selector switch selects one of the supplied FM (main and auxiliary) broadcast radio wave signals, which is of a higher signal level than the other, and the selected signal is supplied to an FM antenna terminal of the AM/FM receiver. Since the FM signal having a higher signal level is always selected by the selector switch in the FM signal reception mode, the FM signal can be received more stably than it is with a single antenna.
The AM broadcast radio wave signals are received by an AM-reception equivalent antenna having an effective length which is equal to the sum of the length L10 of the power supply lines 111 and the length L11 of the defrosting hot wires 102. The received AM broadcast radio wave signals are supplied through the capacitive coupling wire 104 to the inverted T-shaped antenna wire 103.
Because the received AM broadcast radio wave signals are transmitted through the capacitive coupling wire 104, however, the impedance with respect to the AM broadcast radio wave signals is higher than with respect to the FM broadcast radio wave signals, and hence no sufficient gain has been available for the reception of AM broadcast radio wave signals.
The FM (auxiliary) broadcast radio wave signals are received by the AM-reception equivalent antenna described above. The length L10 of the power supply lines 111 is not optimum for the reception of FM broadcast radio wave signals since the length L10 is determined primarily in view of other design considerations. Furthermore, inasmuch as an end of one of the power supply lines 111 is grounded through one of the high-frequency blocking coils 112, the antenna impedance is relatively low for the reception of FM (auxiliary) broadcast radio wave signals, and the gain therefor has not been high enough.
The high-frequency amplifiers 113, 114 are connected to compensate for the reductions in the gains for the reception of AM and FM broadcast radio wave signals. While the high-frequency amplifiers 113, 114 are effective in compensating for the gain reductions, they are responsible for increased white noise, an increase in the cost of manufacture, and a reduction in the sensitivity due to signal saturation in geographic regions where the field strength is high.